Fix $position service to make it work in IE8

It doesn't work when calculating the position of an element while the document is scrolled down

You can verify that using the test.html page: right after loading the page, scroll it down to the bottom and wait 6 seconds; a new element should appear beside the DELAY div. In IE8, with the old code, the new "Positioned with delay" div appear at the top part of the page.